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Identifying WWII reconnaissance plane wing
Between 1944 and 1945, a US Photo Reconnaissance Squadron or (or something alike) did a survey of the whole Chilean territory using the Trimetrogon camera system (228 rolls of film and about 49.000 photographies). This system consisted of three Fairchild K-17 cameras with 6" Metrogon lenses pointing left, right and down.
I'm a glaciologist doing research using those old images and I'm trying to learn more about the mission that carried out the survey. This is not only out of curiosity, as to do topographic reconstruction I need to resolve the lens distortion parameters and I need to know if all images were taken by a single system in a single plane, or if there were multiple systems involved.
The photographs have markings like this:

To interpret these markings a manual of the time gave the example "137PS-3M109-11-V-121" where the information was interpreted as follows
(1) 137PS. Photograph made by 137th Photo Reconnaissance Squadron USAAF.
(2) 3M109. The year is 1943, indicated by the fourth digit of the calendar year, that is, 3 for 1943. The letter M stands for mission, 109 for the mission number
Therefore, from the information in the picture above, the survey in Chile was done by a unit identified as 91PC but I've been unable to find a Photo or Recognnissance squadron/group/wing or whatever that fits this code (See UPDATE at the end below images).
The number "5" stands for year 1945 and 4028 would be the mission number. But I haven't been able to find any information of such mission.
Good progress would be to identify the model of the plane(s) that carried out the survey, but the only clue I have are thousands of pictures porting the tip of the wing. But I don't know if that is possible for such a small clue.
Can anyone identify this plane or has any further information about this surveying mission?
Below some examples of images of the wing:

UPDATE: On the Wikipedia page of the 311th Air Division it mentions the "91st Photographic Mapping Squadron (later 91st Photographic Charting Squadron, 91st Reconnaissance Squadron): 5 October 1944 – 26 August 1946". So, 91st Photographic Charting Squadron seems to fit the "91PC", and the dates match too. Anyway I haven't find much details about the missions performed by this squadron, planes used or any other detail.

Have you contacted both the Army and Air Force to ask them if they have information (long since declassified, I'd imagine) about these missions? I'm not sure which service would have the info now, but that seems like this would be the most accurate source of information for you. They may be able to not only give you a complete "decoder ring" for the text on each image, but give you information about the aircraft and cameras in use.

It looked too short to be a C-47 wing, which meant it was most likely a Beech 18.  And sure enough, in the cropped image below (taken from a copyrighted image - uncropped pic at the link above), you can see the same aileron trim tab arrangement with an access panel just forward of it as the left wing in your images.
So there you go; it's a Beechcraft Model 18, widely used by the US military in WW2 as a utility light cargo, trainer, and short range executive transport airplane.  It makes perfect sense that the photo recon missions you described would use these airplanes.

Thanks for the answer! However, I haven't been able to find any reference of the Trimetrogon system mounted on that plane. I found several references of it been mounted in a B-17, and to me, looking at pictures look more similar than this one. For example https://www.cnn.com/2019/10/02/us/b-17-flying-fortress-bombers-wwii/index.html

I found a reference for Trimetrogon images on a Beech 18, and the images look very similar. I would still like to hear why you discarded the B-17 as an option.

If the camera was mounted in the fuselage, the wing image of the B-17 would be way longer/more slender, and we might see an outboard engine in it. But mainly, the Beech aileron trim tab is near the centre of the aileron, the B17 one is at the inboard edge. So yeah, it does not look like a B17, it does like a Beech 18

A little detail in support of this: the WP page for the 91st (in all its various guises) does briefly mention them operating F-2s, the photo-recon version of the Model 18 (in addition to both B-25/F-10 and B-17/F-9); source seems to be here

Also as you can see here: https://en.wikipedia.org/wiki/Boeing_B-17_Flying_Fortress#/media/File:B17_-Chino_Airshow_2014(framed).jpg  the B-17's trim tab is at the inboard end, and it has a split flap so the wing skin inboard of the aileron is continuous to the trailing edge.


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Do Jabiru aircraft have a long history of engine failures or other technical issues?
I heard that a friend of mine experienced an engine failure in a Jabiru 2200. Does this happen very often and are there other common failures in the Jabiru family of aircraft?

I don't know about a "long history", but the Australian aviation authorities (Jabiru is an Australian company) did introduce operating restrictions because of what they considered to be a high rate of engine failures. Note that Jabiru makes their own engines, unlike most other aircraft manufacturers, and as well as using them in their own aircraft they sell them for use in LSAs.
CASA mentioned 45 incidents in one year:

These precautionary limitations follow a high number of Jabiru engine
  failures and power loss incidents, some of which resulted in aircraft
  forced landings.
More than 45 Jabiru engine failures or in-flight engine incidents have
  been reported in 2014, with CASA recently becoming aware of incidents
  in previous years.
Problems with Jabiru engines include failures of through bolts,
  flywheel bolts and valve train assemblies, as well as cylinder
  cracking.
The failures affect a range of Jabiru engine models and have occurred
  in aircraft used in different flying activities, although many have
  been reported in aircraft used for flying training.

By itself, the number 45 doesn't mean much without considering more information about the number of aircraft, the number of hours flown, maintenance performed etc. and Wikipedia says that Jabiru didn't agree with CASA's decision (which isn't surprising). But presumably CASA did think about those things and still decided that there should be restrictions in place.

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Who decides how fast commercial aircraft fly: the pilots or ATC?
Once a commercial aircraft is in the cruise phase, who determines the speed of the aircraft?
Do pilots have the authority to fly at up to the maximum permissible speed of that plane, or does ATC decide on that depending on the approximate time it will take to reach the destination? Is it stated in the flight plan that the pilot cannot go beyond some specific speed even if the aircraft is capable of it?

Non-pilots often seem to have the idea that controllers are some kind of gods that boss around aircraft. That is not how it works. The controllers are there to assist the pilots, not boss them around. Pilots may follow the recommendations of a controller, but the pilot ALWAYS has the final say and power to decide.

@TylerDurden That statement is correct, but incomplete: When operating under positive control (in the US that generally means class A & B airspace, on an IFR flight plan, or on movement areas of a controlled airfield) pilots are required to obey instructions given to them by ATC, absent an emergency or some other really good reason to justify a deviation from their clearance. While a pilot CAN do whatever they feel like with the aircraft by virtue of being at the controls if they simply ignore ATC clearances they don't like it can lead to some unpleasant conversations with the FAA.

@voretaq7 Agreed, if the pilot actions trigger a review and it turns out he deviated in a way that compromised safety or showed poor judgement, gone goes the license. Keep in mind though that controllers screw up all the time or make blind instructions that are just wrong for some reason, so it is very common for pilots to reject instructions, and I have done so on many occasions.

@TylerDurden It's a bit off topic, but I am reminded of bumper stickers on some cars in airport employee parking lotS in the early 1980s before the controller strike. They said "CONTROLLERS TELL PILOTS WHERE TO GO." The stickers disappeared after the strike was broken.

Yes, pilots are in command: during a DeHavilland Comet flight, Elizabeth von Battenberg was one of the passengers, the pilot wanted exhibiting the airplane's performances, and increased power to top, to gain airspeed; experience was aborted when the flying machine started uncontrollable vibrations...

ATC can assign speeds en-route for separation or in the TMA area for sequencing aircraft onto the approach. Speeds en-route in upper sectors are assigned using Mach numbers:
R: "DLH123, maintain speed Mach .78"

In the TMA area, aircraft are sequenced for an approach and reduced to similar speeds to maintain lateral separation. These restrictions can be paired with conditions, until when the speed needs to be maintained:
R: "BER456, reduce speed 210 knots"
R: "BER456, cleared ILS approach runway 24,
    maintain 170 knots indicated until outer marker / 5 DME"

Certain approach or arrival charts list maximum speeds for certain turns, marked with MAX 210 KIAS, indicating that this turn should not be flown with more than 210 KIAS to make the radius.
Speed restrictions imposed by ATC need to be maintained until cancelled by the ATC unit:
R: "DLH123, no speed restrictions / resume normal speed"

The pilot however is responsible for ensuring a safe flight and can report that he is unable to follow the instructions due to performance limitations.
A: "DLH123, unable to comply, indicated airspeed will be 150 knots"

There is also legal considerations when it comes to aircraft speeds, such as general rules for traffic below certain altitudes or in some jurisdictions, only for certain airspace classes (related: What is the speed limit in European Airspace).
The FAA CFR 91.117 says:

§91.117   Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

The German LuftVO however rules that a speed limit of 250 KIAS below FL100 only applies to VFR and IFR aircraft in airspaces class D,E,F,G and only to VFR aircraft in airspace class C. IFR aircraft in airspace class C do not have a speed limitation.

Regarding the general authority over a flight, see the related question:
Who has the higher authority, the pilot in command or ATC?

Standard ATC phraseology for speed does not include "knots" nor "indicated". KIAS is implied.

@SteveKuo Corrected for the speed assignment, knots is used however according to my AIP. In the advice that one is unable to maintain a speed, the word "indicated" is used.

When on Oceanic crossings with no radio coverage, at the entry point to the track the pilot is given a clearance which includes the speed.  The aircraft is expected, unless safety overrides, to maintain that speed since it is the only way that the controller knows where the aircraft is in relation to other aircraft on the same track.  They can also estimate the aircraft position.

@Simon Should I incorporate that into my answer? Do you have an example/source for phraseology? I am using a German AIP for the most parts, which obviously omits oceanic clearances...

@rbp I added that part about airspace limitations.

i think you can improve this already very good answer by breaking it up into 5 airspeed limitations: routing (charts and plates), airspace (eg US below 10K), operational restrictions (imposed by the airline), ATC clearance, weather (eg temps, turbulence, etc)

oh, and holds! http://en.wikipedia.org/wiki/Holding_%28aeronautics%29#Speed_limits

@rbp weather I am not really an expert to phrase that properly. You want to give it a try and edit my answer?

@SteveKuo Do you have a reference?  In the UK at least, all speed instructions are suffixed with "knots" to ensure no confusion with headings.  E.g. "turn left heading 220 degrees speed 200 knots" is much safer than "turn left heading 220 speed 200".

@SentryRaven I could not find an official reference, but this is a good link.  In this document is an example of a clearance - “ACA865 is cleared to Toronto via Track Bravo 283 from Mimku (56 N 010 W), maintain Flight Level Three Five Zero, Mach Decimal Eight Zero.”  http://www.theairlinepilots.com/forumarchive/areaprocedures/natops/natclearance.pdf

It's worth noting that the 250 kt limit below 10,000 ft in the U.S. does not apply if the minimum safe speed for the aircraft is above that, which is the case for a heavily loaded 747-100 or -200 and probably for the later models as well. Also, if you're icing heavily below 10,000 and expect to be there for awhile, increasing speed to above 250 kts to raise the temperature to stop the icing was normal. Also, if you're below 10,000 and get 12 miles off shore (statute as I remember), the 250 kt limit did not apply prior to the year 2000. I don't know if that's still true.

@Terry  In the UK, the only airspace I know, at the controllers discretion, clearance for speeds higher than 250kts can be given.  Is this true in the US?

@Simon If it's a case of your normal operation being less than 250 but you are capable of exceeding 250, I really don't know. FAR 91.117 (a) says "Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h)." I think, but am not sure, that a controller is acting at the authorization of the Administrator and thus would have the authority to issue such a clearance. You might ask this as a regular question and we'll see if someone knows better?

Pilots will mostly set the speed they want. In cruise, the planes tend to be further apart, and speed is less critical. Different planes fly at different speeds, and ATC usually uses altitude and heading as the primary means to separate them.
However, ATC will sometimes ask for different speeds as another way to separate aircraft. If a fast plane that wants to climb is underneath a slower plane, ATC may have the slower plane slow down a bit, and the faster plane speed up a bit so the faster plane can get far enough ahead to climb.
As the plane nears its destination, speed becomes more likely to be a factor. If there are delays at the destination, ATC may tell the pilots to slow down to minimize time spent in a holding pattern. This may also serve to sequence planes in for arrival, where ATC may ask for faster or slower speeds. Speed restrictions are much more common in arrival, which SentryRaven's answer describes.

ATC is providing collision separation services only. It is the responsability of the pilot to operate the airplane and to keep it under a safe flight envelope. So, speed:pilot, Heading-altitude:ATC :)

This is incorrect: there are many cases where ATC can and does assign a speed (it is extremely common in the terminal environment to ensure apropriate separation on approaches).

speed is commonly assigned by the atc in order to fulfill their aforementioned role :) This is mainly needed on approach, true. Thank you @voretaq7

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How does a Fenestron remove or reduce the tip vortex losses in a helicopter?
How are Fenestrons (or fantails) able to reduce the tip vortex losses in a helicopter?

it's basically a ducted fan: http://aviation.stackexchange.com/questions/13995

A link to some background information would be really helpful here: I have no idea who or what Fenestron is

Which tips, tail or main rotor?  If main rotor, they don't.  If tail rotor, the duplicate covers this.

There are basically two effects here:

One is the ducted fan effect. Ducted fans are more efficient than open fans, specifically creating smaller vortices at the blade tips.
The other is the fact that the tail rotor of a helicopter is not situated in an area with almost linear airflow along the rotor axis. It is also submitted to:

A steady flow due to helicopter forward motion. This is coming from the side of the rotor, so affects opposite rotor blades differently.
A buffeting flow from the h/c main rotor. This is an unsteady flow, also coming laterally from the point of view of the tail rotor (actually, the top).

Fenestron tail-rotor ducts can be considered a fairing that reduces airflow across the rotor. The end result is that the rotor blades are in a much smoother flow that helps reduce unwanted turbulence. This is one of the reasons such rotors are quieter than regular open tail-rotors (the other being the fairing material absorbing some of the noise).


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Approximate Cost of Food & Beverage Preparation & Storage Equipment on A350
What's the approximate price for all the food and beverage preparation & storage equipment on the A350? If you don't know the answer for the A350, I'm interested in hearing knowledgeable estimates with a 90% confidence interval range.
I'm asking for a research project I'm doing.
In response to the critique that the question is too broad: Food and beverage storage and preparation equipment on large wide-bodies is supplied by just 1 or 2 global suppliers. I wouldn't expect the price to be different by geography since it's all typically denominated in USD. But, if it makes a difference, let's say we are talking about the US market.
For an example, I believe I know the following from prior research:
All the galleys in each A350 cost about \$1-1.3 million USD per A350 aircraft,
All the business class seats likely cost about \$2.6 million in each A350 assuming \$70-80 thousand USD per business class seat multiplied by an estimated 35 business class seats for airlines that choose a 2-class configuration of the A350.
All the oxygen systems in each A350 is roughly \$100-150 thousand.
Assuming a 2-class configuration of the most popular version of the A350, what's the approximate total value of all the food and beverage storage and preparation equipment?  If there are important distinctions or configurations I'm not clarifying, I'd ask you to assume the configuration that is most common since that would be most reflective of the overall market.

You mentioned the galleys in your question, what equipment are you asking about that isn't included in the cost of the galleys?

Well, see if you can find the catalogue for the A350. I did a Google search on the galley equipment of the A350 and many pages seem to be pointing at the catalogue which I can't currently find.

I misunderstood your question at the beginning, you're excluding all labor costs (such as meal preparation, transportation and distribution), only equipment purchasing and maintenance. Correct?

@Pondlife: examples of specific equipment would include: coffee and espresso makers, water boilers, beverage containers, refrigerators, freezers, chillers, microwave, convection and steam ovens, etc. And yes, I'm only talking about the cost of purchasing all the equipment that would be needed for one A350. None of the ongoing maintenance cost or the labor involved in actually serving and preparing the food.

This question is too broad.  It might be better if the OP were to ask specific questions about the prices of different foods and beverages (which will depend very heavily on airline), and do the math based off of that.  With the fluctuation of pricing for different kinds of food, and the fluctuation of passengers per plane per carrier...  Estimates for this are going to be ridiculously broad.


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How do helicopters handle P-Factor?
Propeller-powered aircraft have a "problem" called P-Factor, where the descending blade of the prop is generating more lift (a.k.a. thrust) then the ascending blade due to a higher angle of attack when the angle of attack of the wings is high. (I hope I summed that up properly.)
How about helicopters? For this example let's say the blades turn counterclockwise when seen from the top.  When the helicopter is accelerated, the blades on the right side of the helicopter are faster then the ones on the left side.  Thus they should be generating more lift. The higher the speed, the higher the difference in lift on both sides would be.
What mechanism handles corrections that have to be made due to this lift difference? How does it work?

See this and a number of questions on here that already discuss dissymmetry of lift.  Search for "retreating blade" and see the corresponding answers.

Okay, that's a start, I didn't know own that word yet and hence couldn't search for it... Will rephrase my question later, I am more lookin on how  they achieve the corrections made. As they must be different for different speeds and pitch attitudes.

Note that P-Factor is an issue in multi-engine aircraft too

For corrections, see this

This was one of the primary problems that needed to be solved in helicopter flight.
In my example, I'll use a 2 bladed helicopter, but this holds true with all helicopters, and I'll explain some of the different rotor types later on. The blade that is moving forward, relative to the direction of the helicopter, is called the advancing blade. The opposite is called the retreating blade, and you are correct in assuming that they would generate different amounts of lift. As a previous commenter noted, the term used for this is "dissymmetry of lift", and is the same thing that limits the maximum speed of a helicopter.
A two bladed helicopter will generally have what is called a "semi-rigid" rotor system. Semi rigid systems are only possible on even bladed helicopters, and are generally only used on 2 bladed helicopters. The two blades are connected together by a hinge on the rotor hub (where the blade meets the helicopter), and if you push down one blade, the other blade moves upward, and vise versa. What happens as you move forward is that the advancing blade (the one generating more lift) will get pulled up with a greater force than the retreating blade (the one generating less lift). As the advancing blade rises up (because of its additional lift), it pushes the retreating blade down. This advancing blade, in pushing up the rotor, is reducing the angle of attack of the blade against the airflow, which in turn is going to reduce its lift. The retreating blade is hinging downwards, which is increasing its angle of attack against the airflow, which will increase the lift it provides. These two forces will work against each other until they are in balance, which means that the advancing blade and the retreating blade will even out in the lift they are generating, which removes the dissymmetry of lift.
The blades in a helicopter are spinning at a some speed, which varies depending on where on the blade you are talking about (the tip or near the rotor hub, for example), but to simplify this explanation, I'm going to assume that the blade is traveling at the same speed along its entire length. Let's say that the blade is spinning around at 100mph at its tip. As the helicopter moves forward, at let's say 50mph, then we have one blade that is moving against the air at 150mph (the advancing blade), and one that is moving against the air at 50mph (the retreating blade). As the helicopter gets closer to 100mph (the speed of the blades), at some point, the speed of the air against the retreating blade will get closer and closer to 0. When it hits 0, it will stop generating lift, and the helicopter will start to roll (now the rotor is only generating lift on one side). This effect, called retreating blade stall, is one of the reasons why helicopters are limited in their top speed.
Just as a side note, I have greatly simplified some of these explanations, so I am well aware that I have glossed over various parts of this explanation to simplify it.

The blades are hinged and left to flap. Thus, the advancing blade flaps upwards, that flapping motion reduces the angle of attack, and the excessive lift resulting from the stronger relative wind is reduced too. The retreating blade flaps downwards, that flapping motion reduces the angle of attack, and the insufficient lift resulting from the weaker relative wind is increased. In that way, discovered by Juan de la Cierva in the 20s, the lift is conveniently equalized...

You should have read Tom K's answer..


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Can pilots tell if a cloud or fog is made out of water droplets or ice crystals when flying through it?
I understand that clouds can be made out of water droplets or ice crystals. And that fog is nothing else than low-lying clouds.
However, I've experienced fog in a wide ranges of temperatures. I like mountaineering and you often get fog high in the mountains. I've been lucky enough to visit the high mountains of Antarctica and I've also experienced fog there at very low temperatures.
This answer (in the comments) and the fog wikipedia article mention that ice fog happens only at extremely low temperatures (-35 or -40 C). But I think I've experienced fog at those temperatures too. Some times, with the right light I've seen the air filled with tiny crystals, but I wouldn't call that "fog", because it is very thin.
Then, as far as I can tell, all thick fogs/clouds feel and look the same. No matter if they are made out of ice crystals or water droplets. Is that right? Or is there a way to differentiate between ice/water fogs?
Or maybe contrary to what I think, I've never experienced frozen fog.
But, would I notice the difference if I were to ever experience it? How would they feel/look different?
Other sources, like the UK meteorological office say that water always remain in liquid state in fog, no matter how low the temperature is. Is that true? Is it true for clouds too? In that case there would be no such thing as "ice clouds".
One possibility is that ice or water clouds would give rise to different optical phenomena, like sun dogs that are produced by ice crystals, but usually in conditions were they are not dense enough to be considered fog, like in this image:

(Image from Wikipedia page for sun dogs)
So I wonder what's the experience of pilots, as they go through clouds?. When they do so, they know also the external air temperature, which is a plus. Do they see sun dogs or other optical phenomena that differentiate water from ice clouds?

There is also another phenomenon to distinguish: Water droplets can have a negative temperature and turn from water to ice only when they collide with the aircraft (supercooled water)

There may be reflective/refractive qualities of high thin clouds (halo around moon) which may be of some use.  Modern jets also fly above most weather (notable exception thunderstorm cells, which can reach 55k feet).  However, once in cloud visibility is very limited, and can easily drop to near zero.  Visual, and radar, only shows what is reflecting back to you.  What ever is behind that can be hidden.  Clouds themselves change constantly not only from evaporation to condensation, but also again changing state from liquid to solid.  These dynamic "weather makers" are difficult to predict.

Boeing provides some decent notes on this in their powerpoint which you can find in full here. Their sections on pilot reports and the summary section have some good notes for identifying ice crystals in flight/clouds/fog

Pilots have reported rain, sometimes ‘heavy rain’ on the windscreen, Sometimes at impossibly cold temperatures. This is
  believed to be the result of these small ice crystals melting on
  impact with the heated windscreen
No observations of significant airframe icing
Turning on landing lights at night and seeing reflective precipitation .It is likely they were seeing the larger particles even
  though they make up a smaller fraction of the population
Hearing a different sound from rain. Ice crystals hitting the airframe sounded different than rain or hail
Light to moderate turbulence
No significant airframe icing
TAT probe frozen
Ice detector not detecting ice (when installed)
Appearance of rain on the windshield

The same powerpoint also discusses that satellite imagery can be used to see ice crystals. A lot of planes even small stuff have the ability to receive feeds of this data which can be used to identify sections of ice crystals in flight.
According to various sources the threat is particularly dangerous due to the fact that there is no hard and vast way to visually identify the crystals and in many cases the effects are similar to other events and may lead the pilot to belive something else is happening.
For pilots we can only go by look, unless you are flying slow in a 172 you cant exactly pop the window open at altitude and feel the cloud. Simply put pilots can not reasonably tell if a cloud is vapor or crystal prior to flying into it and observing the above noted actions.

So the answer would be that both types of clouds can not be tell apart by visual observation? (and not even with aviation radars)
I read the whole power point and that's what I understand.

@CamiloRada that would be my conclusion as well. It seems most of the incidents eluded investigators and the threat is fairly serious since there is no immediate way to visually identify the issue.

@CamiloRada I have added my comment to the body, if you belive this answer is correct you should accept it.

I gave the bounty to you but I won't accept it yet, because it still doesn't really answer the question, or give a solid answer like "no, they can't". Also, I would guess that not all ice clouds are of the kind that produced engine problems.

@CamiloRada Ill update the answer to reflect but i belive the simple answer is that they "no they can't". Generally answers like that are avoided here if the situation is a bit more case by case which this seems to be.

The telltale sign is observation of ice buildup as supercooled water droplets strike and freeze onto leading edge surfaces.  The probability of this happening increases as one approaches + 32 F (or 0 C), and decreases in colder air ( - 15 C). Freezing rain (rain falling from a warmer layer of air into a freezing layer of air) will also cause ice build up. Yes, it can rain 10 degrees below freezing.
In doing a pre-flight weather check, one can only determine probabilities, as weather (fronts, freeze lines, precipitation) cannot be pin pointed to the nearest mile and are always changing.  However, certain weather patterns such as warm fronts overriding cold air, relative humidity, and near freezing temperatures do provide vital clues.  One would build in a safety margin and avoid if at all possible.
Unfortunately, it is very difficult to tell exactly what is in a cloud until the ice actually starts to form.  Vertical air currents could melt, supercool, and refreeze the same drop of water and a plane flying at the same altitude could experience a variety of "weather" as it passes through. Under these conditions constant observation is a must, as well as a "Plan B" to get out of there if icing compromises safety.

This seem to be to focus on freezing rain conditions, that as well as rain makes very obvious the state of the water in the cloud (water and supercooled water). However the question is about non-precipitating clouds. To distinguish if they are form by suspended ice crystals or water dropplets.

There is a fine line between "precipitating" and "nonprecipitating" clouds, governed by updraft forces.  This is why rain can be a mist, or large hailstones can form.  Certain conditions would increase the probability of one or the other, however, both could exist in the same cloud.  From the pilots point of view, this makes it very difficult to determine.

One benefit of cruising at 35,000 feet is the air temperature is around - 55 Celsius, so yes, it's sun dogs or halos (ice). Lower altitudes (ascent or descent) can be more problematic.  But altitude change does present an alternative to diversion to avoid icing conditions.

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Do fuel reserves take into account the possibility of a constant headwind?
I'm concerned about safety rules for commercial flight in North America. There is supposed to be a fuel reserve of "30 minutes" if I remember correctly. This seems to imply an extra 30 minutes of flying time, but this is not perfectly clear.
Specifically, for example, does this 30 minutes include the worst case scenario of a headwind during the entire flight? If so, what is the max headwind speed used in the calculation?

The "reserve" meant for anything unplanned during the flight is called contingency fuel, which is about 5% of the trip fuel (some exceptions apply).

Fuel needs include forecast winds.   If a headwind is predicted, the pilot needs to plan for that.  If a tailwind is predicted, they account for that as well.   Wind forecasts are usually quite accurate.

The "duplicate of" question does not have the word "wind" in the question nor either answer.  I'm mystified how this can be considered a "duplicate" of the other question, when it is CLEARLY asking for something else beyond "all the rules governing required fuel."  Simple conceptual question about an aspect that IS NOT addressed in the other set of answers.

The fuel "to reach the destination" is computed using winds, head- or tail-wind as appropriate. For "30 minutes of reserve" that is 30 minutes, and it is the same 30 minutes regardless of flying into or with the wind. In 30 minutes of flying with a tailwind you'll fly FARTHER than 30 minutes with a headwind, but 30 minutes is 30 minutes.
-- Edit to answer the question as expanded in a comment:
Wind predictions are actually pretty good now, and "30 minutes" isn't really all that the pilot has to work with.
For FAR Part 91, essentially general aviation (think taking a Cessna on a flight of a few hundred miles as an easy picture), the legal minimum in good weather is 30 minutes.  For FAR Part 121, which governs airline flying, it's 45 minutes (plus other categories as explained in the other answers for things like a destination alternate).  Beyond that, the plan for the flight will almost always include additional fuel above that to account for the routine things that happen to burn more gas: delays waiting to take off, small changes in routing that add a few miles to the planned path, and so on.  That way, even with the routine "stuff" that happens, you don't plan to eat into the 45 minutes reserve.  If the wind forecast is off, then that tends to be another of "those things" that changes the plan.
Realistically, how much are the winds off of the forecasts?  Typically, the forecasts are pretty close.  The computer models that are used now are very detailed, which is possible with all the computing power that's available now as compared to a decade or two ago.  Also, these models are constantly updated by data coming in from aircraft that are airborne.  Various datalinks can transmit the winds and temperature in real time, and ATC will also sometimes ask for current winds at a particular altitude.  All of this combines to give a picture that isn't simply a wind forecast issued in isolation, but a combination of forecast + recent data.
The issue that would arise if an airline were required to plan on the worst-case wind that might be encountered as if it would be present for the entire flight, is that in some cases the flight would significantly under-burn: it would arrive with more fuel than planned.  Why is that an issue?  Maximum Landing Weight.  If your airliner is fully loaded with passengers & cargo, there is a limit to how much fuel it can have on board at landing, driven by the maximum certified landing weight.  On my airplane, that tends to be enough fuel for a bit over two hours of flying.  (Obviously a LOT of factors influence this on each flight -- this is a ballpark number.)  So if I have a full airplane and I want to plan on having as much fuel as possible upon arrival (so that we can hold if necessary, fly the approach, and still have adequate fuel for a missed approach and then a divert, arriving at the alternate with adequate margins), there is a limit to how much I can take off with.  And that limit is driven by how much fuel we're expected to burn enroute.  If my enroute burn is lower than planned and I do NOT have to hold or take a longer route than planned, I can show up too heavy to legally land.
There are two options when that happens: land anyway (safe, but requires an inspection of the aircraft, and it's generally frowned upon to do this without good reason), or fly circles to burn off the gas.  The latter is what is generally done, and is obviously wasteful.  So it's preferred that the enroute burn calculations be as accurate as possible, rather than including unrealistic headwind values.

Yes but how can you be sure that the predicted winds will be the same all along the flight path the whole flight? That's 1,000+ miles and 2+ hours for the typical US flight if I'm not mistaken, and that's a lot of predictions for a lot of ground for a lot of time. Shouldn't they be taking into account the worst case scenario? Plus if I recall correctly from researching wind power, wind is the hardest part of the weather to predict, even harder than rain.

Expanded the answer to address the questions you added in the comment.  You can edit your question if you'd like to incorporate these questions into the original one.

'from researching wind power, wind is the hardest part of the weather to predict' High attitude winds are a lot easier to predict than those closer to the ground. That's one of the reasons for considering high altitude wind generators.

@DrZ214 High altitude wind predictions 3-4 hours out will usually be quite accurate. Even surface wind predictions are pretty accurate on that time scale. Air has momentum, too. It doesn't change velocity without a reason.

@DrZ214 - high altitude winds are pretty consistent, give me the last year's worth of data and I can probably predict next years with a reasonable degree of accuracy. At the very least, they tend not to be entirely surprising even when they deviate from the norm.

winds aloft are forecast based on measurements as well as models: "The data used in these displays come from complex weather computer models that start with the existing weather data from surface weather stations, weather balloons, satellite, and other data and then forecast into the future."

The full regulation reads (S121.639)

Fuel supply: All domestic operations.
No person may dispatch or take off an airplane unless it has enough
  fuel--
(a) To fly to the airport to which it is dispatched;
(b) Thereafter, to fly to and land at the most distant alternate airport
  (where required) for the airport to which dispatched; and
(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption
  or, for certificate holders who are authorized to conduct day VFR
  operations in their operations specifications and who are operating
  nontransport category airplanes type certificated after December 31,
  1964, to fly for 30 minutes at normal cruising fuel consumption for
  day VFR operations.

Part a denotes that they must be able to fly to the airport to which they are dispatched. There is an implied assumption that this includes any and all headwinds in the calculations. Likewise part b requires them to land at the most distant alternate (where required) which would also imply headwinds are calculated for.
Interestingly enough the part 91 regulations (non airline) do specifically state that winds must be accounted for,
Fuel requirements for flight in VFR conditions.

(a) No person may begin a flight in an airplane under VFR conditions
  unless (considering wind and forecast weather conditions) there is
  enough fuel to fly to the first point of intended landing and,
  assuming normal cruising speed--
(1) During the day, to fly after that for at least 30 minutes; or
(2) At night, to fly after that for at least 45 minutes.
(b) No person may begin a flight in a rotorcraft
  under VFR conditions unless (considering wind and forecast weather
  conditions) there is enough fuel to fly to the first point of intended
  landing and, assuming normal cruising speed, to fly after that for at
  least 20 minutes.

The required fuel reserve is for 45 minutes. According to 14 CFR 121 §121.639   Fuel supply: All domestic operations:

No person may dispatch or take off an airplane unless it has enough fuel—
(a) To fly to the airport to which it is dispatched;
(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and
(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.

The factors to be considered in calculating the fuel reserve include the winds. According to 14 CFR 121 §121.647 Factors for computing fuel required.

Each person computing fuel required for the purposes of this subpart shall consider the following:
(a) Wind and other weather conditions forecast.
(b) Anticipated traffic delays.
(c) One instrument approach and possible missed approach at destination.
(d) Any other conditions that may delay landing of the aircraft.

This explicitly states that any headwind experienced now or is expected based on weather forecasts is to be included in calculating the fuel quantity required.
Note that the ICAO annex 6, part I 4.3.6
Fuel and oil supply  calls for similar fuel reserves, stating,

A flight shall not be commenced unless, taking into account both the meteorological conditions and any delays that are expected in flight, the aeroplane carries sufficient fuel and oil to ensure that it can safely complete the flight. In addition, a reserve shall be carried to provide for contingencies.

In all these cases, headwinds enter into calculation only if the the present weather conditions or forecasts indicate their presence. However, in general, the fuel reserve carried is generally more than the 45 minute reserve, for safety.


text:
What is the "A/B AUTO EXT" button for in a Falcon 7X?
What is the "A/B AUTO EXT" button in the overhead panel of the Dassault Falcon 7X? I see it's in the Flight Control section but I don't know what is its purpose.

(sorry for low quality image)

What happens when you press it?

That cracked me up....

It's the arming switch for Air Brake Auto Extend on landing.  The airplane has speed brakes (often called Flight Spoilers) the pilot can operate manually for glide path control.  This mode extends them automatically as lift dumpers after touchdown.

I see! It's an unusual position on the overheal panel (and an unusual name). Thank you.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is a controller at a NFCT required to have an FAA physical? If so what class?
Is there a mandatory requirement for a Tower Controller working at a Non-Federal Control Tower (NFCT) to have a current FAA medical certificate?  Is a Class III sufficient?

Yes, you would need a current medical certificate. Source is here (page 4).
Although the above document is for NFCT, they do not mention which class is the minimum. However, I found at other places that for ATC, you do require Class II (sources are here and here).

Second-Class - Commercial Pilot; Flight Engineer; Flight Navigator; or
  Air Traffic Control Tower Operator. (Note: This category of air
  traffic controller does not include FAA employee air traffic control
  specialists).

This flight school indicates the same:

Applicants must:
  possess a current, Class-2 medical certificate issued through an FAA certified examiner


text:
Is LHV or HHV used for calculating the thrust of a jet engine?
Lower and Higher Heat Values (LHV or HHV) seem to have multiple definitions based on things I can't figure out.
What I want to know is, which one is appropriate to use for calculating the thrust from a jet engine? I'm thinking of large commercial turbofan engines like on the 747 or A380.
Also, since water vapor seems to have something to do with it, what about jet engines that run on hydrogen like the experimental Tu-155? Should we use LHV or HHV to calculate thrust in those engines?

Use for what? Those are just numbers that describe the fuel.

I'm voting to close this question as off-topic because it is more about thermodynamic calculations than aviation and as such is better suited for http://physics.stackexchange.com/. After clarification what you want to use the specific heat of combustion for.

On a side note, fuel suppliers usually use the lower heat values. It then leads to paradoxes like natural gas boilers with 109% efficiency.

@JanHudec I'm not sure what is unclear. I want to use it for jet engines, for thrust.

Well, the thrust just is. It is generated by burning the fuel, not by chewing up numbers! You don't need to use numbers for it. Now if you want to calculate thrust, that's another matter, but you didn't say that.

Now that we've established that you want to calculate thrust, you can use either! You just have to be aware of their definition and of the fact, that the water is still in gaseous phase and include specific heat of evaporation or not as appropriate for the heating value you use.

@JanHudec yes I want to calculate thrust. I honestly don't understand what else you're saying. Thrust and pretty much everything else obeys natural laws. I want to know which law is at work, one involving HHV or one involving LHV.

If you were instead calculating efficiency, you'd get different efficiency for different definitions of heating values. And while it's the LHV-based one that is normally used, the HHV-based one arguably makes more sense. But for thrust (via heat of the combustion gas) you can use either definition, if you use appropriate equation.

For calculating thrust in the usual manner using some energy balance across the combustion chamber, the LHV needs to be used. I think that is pretty simple and straightforward and Peter Kämpf answered it well.

@JanHudec Why does the HHV make more sense to you? Unless the device has a way to condense the water vapor evolved how is the HHV equivalent energy ever accessible in an aircraft type jet engine?

@JanHudec Do you have a link or any more info about the 109% boiler efficiency case you cited? Just curious. Because to get 109% based on LHV the natural gas boiler would have to be a condensing type. Do such boilers exist in typical use? I'm used to coal fired boilers and ours tend to be in the 88% efficiency range, again based on the LHV.

@curious_cat: Yes, a condensing type. Yes, they exist. I have such thing at home. Basically in last maybe 10 years if you needed a new one, there is little point in not getting a condensing type. I believe the 109% was mentioned in the specs as maximum under favourable conditions, but I can't quickly find English Junkers page (their .com domain is in German).

The difference between both values is the energy set free when the water created by the combustion process condenses. Use the lower heat value for jet engine calculations. The water in a jet engine leaves it as vapor, and the vaporisation heat is lost for propulsion.


text:
Can I file IFR DIRECT with a non-WAAS GPS?
The AIM 1-1-17 2 (a) states:

Aircraft using un-augmented GPS (TSO-C129() or TSO-C196()) for
  navigation under IFR must be equipped with an alternate approved and
  operational means of navigation suitable for navigating the proposed
  route of flight. (Examples of alternate navigation equipment include
  VOR or DME/DME/IRU capability). Active monitoring of alternative
  navigation equipment is not required when RAIM is available for
  integrity monitoring. Active monitoring of an alternate means of
  navigation is required when the GPS RAIM capability is lost.

Does this mean that if I have a non-WAAS GPS unit then I have to file only along Victor airways or between VORs? Or can I file DIRECT?

Giving up a little on my age here.  A long time ago, I was on a return flight home at FL250 in a Seneca II. On an IFR flight plan using VORs. ATC asked me if my LORAN was working. My reply was yes, but not certified for IFR.  ATC then asked if I could tell him what my initial heading would be for my destination, I replied with the information.  ATC then cleared me direct to home.

This question was answered here on this Website.  Read the second answer.
Yes you can file direct even without a GPS.  However it would be advisable to perform a RAIM check during your pre-flight along your intended route and determine that you would be in RAIM during the route.  If for some reason your GPS loses RAIM capability then you should proceed using VOR/DME/ADF.  You can still fly direct with using only VORs although that will be more difficult.

In my opinion, navigating an IFR flight plan is more than just the ability to get from one airport to the other.  I am also concerned about communication radio failure.  If you follow the advice listed in the AskACFI website, you will not have many options if your communication radio fails
It is this reason, why service volumes (see AIM 1-1-8) for VOR and NDBs are published.  Assuming there are no restrictions listed in the chart supplements booklet, you can fly from VOR/NDB to VOR/NDB as long as you stay within the service volume.  If you lose communication radios, at the very least you will be able to revert to the requirements listed in FAR §91.185.
I once filed TUS to IWA, a total distance of 81.4 NM.  I would have been out of the service volume for 1.4NM and ATC would not give me that clearance.  Go figure.

On your TUS to IWA flight, if your aircraft is GNSS equipped, direct point-to-point is no problem. If it's /A, I would issue KTUS.V16.TOTEC..IWA..KIWA, then give you a vector to IWA VOR when I had you in radar.

@atc_ceedee.  They issue V105 because V105 is used for flights from PHX to TUS.  I agree with your comment and makes complete sense.  Welcome to the forum.  Your knowledge is quite valuable here.

Ah, that makes sense. As far as the communication issue goes, though, ATC should issue lost comm instructions (although, I admit it's nearly a lost art) for any clearance where an aircraft may enter areas of poor communication. The onus there is on the controller (we're supposed to know our frequency coverage), but that's poor consolation if you find yourself out of comm range and in trouble.

You may file a direct route with no more than a radio, and by doing so, you're telling the controller that you can actually fly it.
Controllers have their own rules, though, which are there to protect you and the FAA.
For example, if your aircraft is /X equipped, and you file a route beyond navaid limitations, it's my responsibility to ensure that you are cleared on a route that meets those limitations. There is no gray area.
(Oddly, a Center controller can use anticipated radar coverage for separation, but not for eliminating navaid limitations. Maybe terminal has a rule they can use.)
Once you're in radar contact, however, I can clear you direct to your destination, and radar monitor your flight (generally, I'll clear you direct the navaid that services the airport).
If you get off course, I can then provide navigational assistance (vectors) to get you back on course.
You may also file OTP (VFR on top), and receive clearance beyond navaid limitations, without the necessity of being in radar coverage. For my part, if you are out of radar coverage, I'll ask for position reports every so often, to make sure we're still in contact, and that you're still alive and kicking.

Since you are a retired controller, I wonder if you would have anything to say about these questions --  , https://aviation.stackexchange.com/questions/64205/in-the-us-in-actual-practice-workload-permitting-will-artc-facilities-grant-s? , https://aviation.stackexchange.com/questions/48106/does-an-svfr-clearance-extend-to-echo-surface-extensions .  Btw, I now consider my answer to the second one to be in need of a great deal of revision.Here's another related one-- https://aviation.stackexchange.com/questions/49063/is-the-official-weather-report-for-an-airport-in-class-d-airspace-controlling-fo

Regarding "SVFR and class E extensions", Center is required to keep all IFR and other SVFR aircraft clear of the entire Class E area for the airport until the SVFR with clearance is either on the ground or clear of the surface area. If you need the airspace, you have it. In the case of Class D, the tower must be coordinated with (usually, THEY make the call, but someone has to) and then the clearance may be issued. We will issue altitude based restrictions if necessary (at or below ALTITUDE while in Class E...) to separate from IFR traffic.

Regarding "weather report in Class D controlling Class E extension", I think pondlife's answer is great. If, for instance, an aircraft wants to traverse the area in SVFR conditions, I'd allow it based on the weather, if it meets SVFR minima. If the weather is below those minima, I'll ask the pilot about his flight conditions, and to say intentions. If the airport is busy, I will probably deny clearance. They should go around. If, however, the pilot elects to go THROUGH the area anyway (and I don't know which target he is), there's not much I can do except traffic advisories to other aircraft.

A good example, on the Cheyenne sectional chart, would be STK and AKO airports, which have surface areas that overlap. If an aircraft requests SVFR into KSTK, and I issue it, I'll protect that KAKO area, as well. In this instance, it's very unlikely (based on normal traffic) that these two airports would be active at the same time, anyway.

The requirements for SVFR constrain the controller to protect the entire area because the aircraft must maintain visibility and clear of clouds minima. I don't know what the pilot must do in a given situation to accomplish that. That is why we sterilize the airspace when we clear SVFR. Conversely, I can clear two IFR aircraft into/out of/through Class E areas, because I can constrain them to headings, radials, courses and altitudes, etc.

I understand the operational need to keep IFR traffic out of the entire "surface area" (and I'm using the phrase in this case to include the extensions) if any SVFR traffic is there, but less clear is whether SVFR ought to be authorized in the "extensions" at all.  I'd be interested in your response to my newly-posted answer https://aviation.stackexchange.com/a/86956/34686 .  (Since we're not to use comments for discussion on ASE, this may end up going to a chat room, which is fine-- )

Re " If you need the airspace, you have it"-- anyway I read this as saying that in your view and actual experience, SVFR generally may be authorized in the "extensions", ie is not prohibited.

Let us continue this discussion in chat.

Yes, keeping an eye on the chat room.


text:
Does the downwash created by induced drag increase or decrease lift?
According to the theory I know, lift is produced by accelerating air downwards at the trailing edge (downwash). Hence, as downwash increases so does lift. Induced drag is due to vortices at the wing tips and they also create downwash. So according to the above theory induced drag should also increase lift.
But, the books say that induced drag increases downwash and reduces the effective AOA, which subsequently reduces the lift. Does the downwash from induced drag increase or decrease lift?

The induced drag is due to vortex at wing tip and it generates a downwash. Hence, as per above theory it should produce more lift. logical fallacy. lift  is produced if THE WING accelerates air downwards. Downwash is not the wing.

It is not "this causes that" - all is happening together. Let me explain:
For me to understand aerodynamics, it helped to disregard all that talk of vortices and induction, but focus on the pressure field around a wing. When the theory of flight was developed, electricity was new and exciting, and it just happened that electric induction could be transferred to lift. Now every author still copies the explanations from a century ago, but they are totally unintuitive.
Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects. Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise. Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they experience. Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.
Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly, the packet will change course and follow the wing's contour, but not without spreading out (= pressure loss). Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.
A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.
What is lift?
Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.
Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.
Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!
Induced drag
The misconception about those "wingtip vortices" and induced drag is hard to eradicate. Most authors copy what has been written before without clearly understanding the issue. Therefore I repeat it here again: Induced drag is the backward-pointing part of the pressure force vector. The vortices are only a consequence of downwash, which in turn is a consequence of lift creation. At the same speed, more induced drag is indeed linked to more lift, but the causality is different: Lift and induced drag are both part of the pressures acting on the wing. If you add up all the pressure forces acting on a wing, their resulting vector will point slightly backwards. The streamwise component is drag, and the component orthogonal to the direction of movement is lift. This is just a defininion, made for simplicity.

If I ever meet you in real life I'm buying you a beer.

This is a servicable explanation but if you want to be strict, there are some inaccuracies: pressure drag is something that also happens on a symmmetric airfoil at zero incidence (i.e. without lift). Only in perfect inviscid flow would the pressure force be equal to the "induced" drag. And while even a theoretical infinite wing (no tips!) carries this type of drag, there is additional energy in the wingtip vortices, which comes out of the aircraft engines, because a finite wing doesn't just push air down but just outboard of the wing tips it's also pushing it up ...

@Zak Phew! Thank you for clearing this up! Gliders don't suffer drag from wingtip vortices because they lack engines. Now everything becomes clear.

@Peter Kämpf: Yep! Another method is to form the wings into a ring, so they have no tips, or only fly in the dark, so nobody sees them :)  And to clear up my ambiguous phrasing: The kinetic energy in the vortices is lost from the plane's kinetic energy. Whether you counter that by producing thrust, gliding downwards or using upwind does of course not matter. Guess it stands to reason that my nitpicking would be nitpicked ... the point was that pressure drag !=induced drag, unless you're flying in potential flow.

@Zak: Agreed, friction adds its own pressure drag component. However, I don't get why you nitpick things I never said, like that a symmetric airfoil at zero lift has no pressure drag. That is something you have made up.

@PeterKampf,what do you think about this explanation?https://aviation.stackexchange.com/questions/77778/how-pressure-fields-change-around-wing

@Сократ Lift is per definition the part of the aerodynamic vector that isn't drag. So induced drag is the backward-pointing part of the aerodynamic vector, excluding friction drag. In oder to exclude friction, I used the term "pressure force vector".

@Сократ. Yes, but. With increasing angle of attack the friction drag also changes and when separation starts you get plenty more pressure drag which is not induced drag. Funny fact: If you assume a linear lift increase even after the onset of separation (which reduces the lift curve slope), drag will still follow the lift-squared function much better than if you calculate drag based on real lift.

@Сократ Yes, induced drag is pressure drag. Now I was thinking of the quadratic drag formula which can be used to separate only that part of pressure drag that is directly caused by lift. Pressure drag from separation is only indirectly related to lift.

@PeterKämpf backward-pointing part of the pressure force vector.  You conncted pressure drag and induced drag in one pressure drag? Aren't they two separte pressure drag?

@JurgenM Induced drag is the part of pressure drag which you get also in inviscid flow. Viscosity adds more pressure drag, e.g. from the pressure changes caused by separation. You are right, this is all pressure drag, but its parts can be cleanly attributed to different causes. Vielleicht wird es klarer, wenn wir auf Deutsch weitersprechen.

@PeterKämpf Was induced drag first time discoverd in lifting line theory by Ludwing Prandl or  was known before Prandl?

@user628075 I have no idea. But I guess it was the result of several small steps. I am sure there was no moment when someone formulated the definition for induced drag out of thin air. Lilienthal already knew that drag goes up with the square of lift, but you will not find the term "induziert" in his book Der Vogelflug als Grundlage der Fliegekunst.

@PeterKämpf I always thought that Prandl first formulated/discoverd induced drag. I think in time of George Cayley induced drag was not formulated..

@user628075 Could very well be, but I cannot confirm it. However, I am in full agreement concerning Cayley. His contribution to aviation is miniscule. Without him the field would had developed in just the same way.

@PeterKämpf Who you consider that have biggest contribution to aerodynamics? Theodore van Karman, Navier-Stokes, Bernulli.....etc.?

@user628075 Hands down, this goes to Ludwig Prandtl. None of your candidates comes close.

@PeterKämpf Ludwing work is based on what  Lanchester discoverd...

I believe the answer given by Peter Kampf is confusing pressure drag with induced drag.
Pressure drag is, as he said, a result of the pressure force acting on the wing having a horizontal component(drag) as well as a vertical component(lift).
Induced drag is primarily down to the effect that the wingtip vortices have on the airflow.
The vortices make the air ahead of it, i.e. the air on top of the wings, increase their velocity downwards. This has the effect of negative lift or downforce. This means for the same amount of lift the pilot must increase the angle of attack.
However, this also means drag has increased as the pilot has increased the angle of attack. This increase in drag is what is called induced drag.
The formula for the coefficient of induced drag is $Cl^2/(\pi \cdot AR \cdot e)$ where $Cl$ is the coefficient of lift, $AR$ is the aspect ratio and $e$ is the Oswald efficiency which relates the shape of the wing to how much induced drag it creates.
From here you can see that increasing the speed of the aircraft reduces induced drag, as for the same lift an aircraft travelling faster requires a lower $Cl$.  Increasing $AR$ reduces induced drag as having longer wings means the wingtip vortices are further away so less of the wing is being affected strongly by the vortices.
And $e$ is just depended on your wing shape. The optimum value of $e$ is 1 for an elliptically loaded wing.

Induced drag is pressure drag. There is nothing to confuse.

There are actually two sources of induced drag (see wikipedia article.  One source is described well by Peter's post.
But the other source is vortex drag, from the wingtip vortices (turbulence), which depend on the aspect ratio of the wing as well as on wingtip treatments like winglets.   In the end the wingtip vortices lower the glide ratio (L/D) of the aircraft.  So if they produce a little bit of lift, it doesn't help because the drag has increased by a bigger factor, lowering the L/D.

You clearly didn't understand the Peter's post. There are no two sources of induced drag, just one. It's all just different aspect of the same process.

Correlation is not causation. Think of it this way. The airflow characteristics leaving the trailing edge (the amount and quality of turbulence present in the wake, etc.) are influenced by the airfoil and the aspect ratio, but the wake airflow characteristics themselves have no influence on anything happening forward in the airstream. The pressure distribution over the upper surface of the wing that results in induced drag (i.e. the lift-vector being tilted slightly backwards) isn't caused by anything occurring behind the trailing edge.

I think I did understand his post but disagreed.  His explanation talks only about the lift vector being tilted slightly backwards, and that part is called drag.  That occurs for all wings, even infinite ones.  But the wing-end vortices occur for only finite wings, and this is the part of induced drag you can get rid of.

No, Bret, the lift vector of any wing is exactly perpendicular to the flow direction at infinity ahead of that wing. This is how the direction of the lift vector is defined. The sum of all pressure forces is slightly pointing backwards for a finite wing, and it is exactly parallel to the lift vector for infinite wings in inviscid flow.

@PeterKämpf   Isac Newton, without him ,no physics.... :)

@user628075 Nonsense! Newton's contributions are real, but completely overhyped by English jingoism. Since the US has adopted the English language, myths have been repeated endlessly and now are taken as truth. Besides, personally, Newton was a jerk.


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Can laser designated weapons such as AGM be used for air to air combat?
Just wondering because I know AGM cant pull any g's but if the target is straight ahead of you and they cant see you i think AGM would be more effective because of Radar lock ping.

Note that most AGMs are guided via Lasers and not via Radar

While possible (a 2000lb laser-guided bomb once took out a Mi-24 Hind), it would not be as practical as conventional air-to-air missiles.
A problem with laser-guided weapons is that you have to limit your maneuvers in order to keep the laser on the target, instead of fire-and-forget weapons where you can do whatever is appropriate next.
Visibility is also a problem - Radar works though clouds, lasers are much more limited.

Good find on that story

Not really. Someone or some thing would have to maintain a laser point on the target for the missile to track to. With ground targets, this is comparatively easy. They don't move that fast, if at all. The target is lased by a wingman, or even a person on the ground.
An aircraft would be continuously jinking.
Better to let the missile guide itself, via IR or radar. The shooter can then go off and do something else.

Air-to-ground missiles (AGMs) can't pull a lot of Gs because they are designed and built to track and destroy ground targets, so why spend the money, (and reduce other performance capabilities), by designing them to pull high Gs? Maneuverability is required in order to react to maneuvers by a target. An aerborne target, is generally capable of maneuvering at high G-Loads, at medium to high velocity. This is what requires an intercepting missile to be able to maneuver at high G-Loads.
The second characteristic is driven by the target's apparent Line-Of-Sight rate (LOS rate). A target with a medium (on or near 90 degrees) of aspect or heading crossing angle, requires the greatest amount of Lead. In order to intercept the target, most missile guidance systems use what is called Proportional Navigation, which basically means the missile pulls lead until it is on the collision course (measured by the lead required to cause the LOS rate to be zero. The faster the missile to target velocity ratio, the less lead is required, the slower the missile or the faster the target, the more lead is required. All missiles seeker heads (the part that has to point directly at the target to track it), have gymbal limits. If the missile can only point its seeker head a maximum of 30 degrees away from its boresight, then if it needs 31 degrees of lead it goes dumb (breaks track and goes ballistic). This was a major cause of launch envelope restrictions for early AIM-9 missiles.
Because AGMs are designed against ground targets, which are generally not moving, or moving at considerably slower speeds than the missile, they do not need to turn to lead the target by very much and therefore generally do not have very large gymbal limits.
So, if you are thinking of using an AGM against a slow, non-maneuvering aircraft at very low (tail-on) aspect angles, where little or no lead is required, an AGM might be able to hack it, otherwise, no.


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Can you fly a helicopter without a driver's license?
Assuming one  does not have a driver's license, is it possible for one to obtain a pilot's license to fly a helicopter? Cost is not the question here. I want to know if one can never obtain a driver's license (for ground, motor vehicles) and still be able to obtain a pilot's license to fly helicopters, planes, etc.
This is applicable for the U.S. only.

This was the case for e.g. Neil Armstrong.

Driver's license is for driving a land based vehicle. It is not require for flying an aircraft.

I help a UK PPL for more than four years before I got a drivers license.

In the United States they only ask you for one thing to start flying helicopters: money. A drivers license is never mentioned. If you are in the marines, they don't ask for the money.

@TylerDurden And identification.

Here's a a video where a CFI discusses this very question. There's no rule that says you need a driver's license to obtain a certificate to fly in the US. The real question is why you can't obtain a driver's license. If you have a medical problem that disqualifies you from driving, that will likely pose a far larger issue getting a medical to fly. Similarly, any drug or alcohol problem or conviction must be reported to the FAA. If your driver's license was suspended or revoked due to drug or alcohol, that must be reported as well. These issues are reviewed on a case-by-case basis, as the FAA wants to be satisfied that you will not try to fly while impaired. Beyond those types of problems, if you simply don't own a car and never wanted to learn to drive, nothing is stopping you from getting a license to fly.
Note that you will need some sort of government-issued photo ID. A passport or state-issued non-driver ID should be fine. You might also run into the practical problem that it may be difficult to get to/from airports in many regions without driving, so you'd have to find alternate transportation.
Finally, there is one sort of connection between driver's licenses and flying in the US. Pilots with a valid driver's license can fly Light Sport Aircraft without obtaining the otherwise-required third class medical certificate. A few rotorcraft (like this one) qualify as LSA. There are, however, significant restrictions on these operations, such as the requirement that they can't fly at night or carry more than one passenger. This rule does not override the medical requirements to fly, but requires the pilot to certify him/herself able to fly safely instead of receiving that certification from a Aviation Medical Examiner. In other words, if there's a reason you wouldn't be able to obtain a third class medical, then you are not allowed to fly a LSA using only a driver's license.

Hello. The reason and details cannot be publicly disclosed, but I am an ex A.W.O.L. soldier from the U.S. Marines. A court marshal ordered that I can't ever obtain a driver's license.

Well that's a new one. It would probably be best to consult with an aviation attorney if you're serious about pursuing this. A military lawyer might eventually be required if you ended up needing to seek modification of the court order. Even if you were able to receive a PPL, your background could well preclude you for flying for any commercial purposes and could pose problems if you went all the way to ATP in the future. It would be best to get legal advice and fully understand the situation before jumping into training.

@HairInAComa I'm a bit late to the party, but no USMC MARINE would stoop low enough to call themselves a "soldier". I'm called foul.

In order to obtain a PPL in a glider, one only needs to be 16, which is younger than the age to get a driver's license in many states. For airplane and heli, it's 17, and there's no evidence that every 17-year-old has such a license.  So there doesn't seem to be any correlation between an actual drivers license and a pilots license, so long as the reasons for not having one aren't in violation of any other laws, regs, or decrees to which you're subject.  You must still provide appropriate identification.
The two applicable FARs regarding disqualified for non-FAA-related legal or regulatory actions are 61.15/.16, which refer to alcohol and drugs, and 61.18, which refers to security disqualification. 15 and 16 don't apply, as you wrote.
61.18, however, has to do with security threats. If the Transportation Security Administration (TSA) has notified the FAA in writing that you pose a security threat, then you would also be disqualified.
If none of these apply, then you (or perhaps better, an aviation lawyer) can write the FAA's chief counsel and ask for an opinion. The relevant contacts are here

In many states the minimum age for a driver's license is 16, so it would be the same in many instances, not younger


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At what age may a student log time and use that time to obtain a rating?
A few years back the FAA was upset about young pilots attempting to set records and crashing in the process.
Were there any rules instituted regarding logging of time with a CFI / instructor? May a 9 year old or 12 year old log time flown with a CFI and use that time to qualify for a rating?

Hello Zeus, welcome to Aviation.SE. Which jurisdiction are you interested to hear the answers for, as this may vary from country to country?

FAA rules in USA.

I did not look into regs yet, but I know of kids that take lessons at that age (which sure isn't illegal) and log those so they can do their first solo (which requires a minimum age) ASAP. this goes for many countries btw,in Germany you can fly gliders solo with 14, but you need to be 16 to get your license...

More than just the FAA - after the death of 7 year old Jessica Dubroff (http://en.m.wikipedia.org/wiki/Jessica_Dubroff), Clinton introduced the Child Pilot Safety act, prohibiting anybody from attempting aviation records without already holding a licence and medical.

I know a CFI friend, who's 4 year old holds 10 hrs dual in his logbook.

FAR § 61.83 states:

To be eligible for a student pilot certificate, an applicant must:
(a) Be at least 16 years of age for other than the operation of a glider or balloon.
(b) Be at least 14 years of age for the operation of a glider or balloon.
(c) Be able to read, speak, write, and understand the English language. [...]

A student pilot certificate enables you to fly a plane solo or on a solo cross country.
In other words, you need to be at least 16 years old to fly a plane (non glider/balloon) solo.

Now, back to your initial question. If you go through FAR §61.83 - §61.95 you will find all the regulations about student pilots (which would be the kind of pilot you are referring to ).
I have found no age restriction going through them for the pre-solo instruction. All it says that it must be done before soloing which does require a medical exam and certificate (but these don't have age restrictions either).
Summing up, a 9 or 12 year old child is legally able to take flying lessons with an instructor, but may not solo before his/her 16th birthday (no glider/balloons).

You might want to clarify what "you must have a medical before starting your training" means. It conflicts with your statement that there's no age restriction for pre-solo training.

I knew someone who logged time before they were 16 but soloed and got their license after their birthday. I can confirm that this will "fly" with the FAA.

authors: มานพ เกิดพิทักษ์


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Do flight simulators imitate turbulence effectively?
I know that flight simulators (which are used to train the pilots for commercial airlines) try to simulate almost all kind of weather conditions, but how effectively and realistically are they able to reproduce turbulence in the simulated environment?

What kind of flight simulators?  There are many.  No PC or FMC simulator I am aware of simulate turbulence and the only full size simulator I've flown, a 737NG, didn't either.

Specifically used by airlines to train the pilots for commercial operations..

Many weather phenomenons are not really well known, especially winds. It would be difficult to recreate them digitally (the very dangerous microburst existence has been unknown till recently). This can be compared to rogue waves which accounts from mariners have been challenged until recently.

No, simulators cannot realistically simulate turbulence. Turbulence is characterized by significant changes in altitude that happen very quickly, and even full motion simulators aren't going to be able to re-create that. They can bump you around a bit, but not really prepare you for the real thing.

Why can't they prepare you for the real thing?

@Koyovis, real turbulence involves accelerations beyond the capabilities of the simulator's motion controls.  For example, a bad bump might produce negative g forces throwing you towards the ceiling -- you can only get that in a simulator by pulling it down faster than gravity, which requires extremely long, fast-acting actuators.

@Mark Electric motion systems can reproduce up to -0.87 g in heave. The resulting specific force of 1 - 0.87 = 0.13g is indeed not negative, but is a pretty serious acceleration, definitely a more memorable experience than just being bumped around a bit. That latter statement is correct for older type hydraulic systems - helicopter simulators would often feature a seat shaker or cabin shaker, which shakes you around quire perceptively, I can tell you.

It depends what you consider realistic. A full motion simulator can slam you around pretty fearsomely, but there is always the limited travel of the motion actuators to consider of course. As opposed to the real world where the amplitude is infinite.
Simulators can realistically simulate turbulence, as in a vibrating aircraft. Modern Level D simulators on electrical motion systems can reproduce vibrations between 0 - 20 Hz at a teeth rattling amplitude. They can also produce downward accelerations at over 1g, making the occupants being ejected upwards from their seats. This can be used to simulate high altitude wind shear effects, while making the altimeters tumble.
Motion systems can bump you around a lot. I'm not sure what the training value of slammin a flight crew through full turbulence would be though - would they pick up any extra skills that could prevent an accident?

There's a study done by an RAF pilot who flies approaches in a sim, with and without motion. The approaches with motion are much more accurate: motion stimulates the acceleration sensors in our head,and the direct sensing is much faster that the info from our peripheral vision.


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Can FMC be programmed to do fast maneuvering in fighter jets?
As we know we set the flight path etc in FMC but can it be pre programmed to do high speed maneuvering and shoot the ammo as per the preset conditions?
It could be kind of self moving UAV with the pre stated path, waypoints, targets to hit and the return path along with the type of ammo to hit depending upon the kind of target.

The capabilities of current military UAVs are classified, so it is not likely anybody is going to give you a definitive answer. On one hand, programming computer to do such things is certainly conceivable, but on the other hand you want a human to confirm any weapon release, because errors tend to have serious political consequences.

Pre-programmed high speed maneuvering is already being done in the case of target drones derived from fighter aircraft like the QF-16 (which is derived from F-16). According to multiple reports,

The QF-16 ... is capable of pre-programmed maneuvers such as high-G turns, barrel rolls, split-S, etc.

While some research has been carried out on fully autonomous weapons systems, at present there is always a human being in the loop (at-least) for pressing the 'kill switch'. A few of the problems in a fully autonomous system operating in enemy airspace are:

Operating the UAV in hostile area and responding to threats. Though the QF-16 drones can fire chaff and flares to escape from incoming missiles, drones that can operate in hostile environment are still in their infancy. This requires significant improvements in ability of the UAVs to detect, differentiate and prioritize the threats and respond to them.
The UAVs should have the ability to search/track, acquire and engage the target while not ignoring friendlies or neutrals. Though some tests have been carried involving automated target tracking and some missile do have the ability to discriminate between the target and surrounding buildings (based on prior information),the technology is still some way off.
There are significant political, legal and ethical issues in using automated killing machines.

Some missiles like the Tomahawk has the ability to navigate along a predetermined path and hit targets, though they cannot do high speed maneuvers (or return, obviously).


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Do large aircraft have carbon monoxide detectors?
Do big aircraft (e.g. Boeing 737 or Airbus 320) have carbon monoxide (CO) detectors installed in them? I don't know if the pressurized cabin means they aren't needed, but I do remember there are CO detectors in small planes and there are some reported accidents due to inhaling CO from engines.

Haven't seen any. Cabin heating is done quite differently on jet aircraft than on pistons. Bleed air is taken from the engine before the combustion chamber, so engine exhaust can't end up in the cabin, even if there was a greater fault in the A/C packs.

There are things that can get into the AC ducts by way of the packs, but CO isn't one of them.  I've had hydraulic fluid from an overserviced resevoir get into an AC pack through a torn seal, and that stank up the cabin pretty bad, and any FOD (like a bird) that gets into the engine's bleed air system will go through the pack and come out smelling bad, but CO, as a product of combustion, happens downstream of where the bleed air for the AC is taken out.  I've never seen a CO detector in a large turbine aircraft either.

It doesn't have to be exhaust gases- there is potential for smoldering small amounts of insulation.


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On what principle(s) do aircraft smoke detectors work?
I know there are smoke detectors in aircraft but on what principle(s) do they generally work?
Commercial smoke detectors can use several different techniques like optical sensing, heat sensing and particle sensing. Do aircraft smoke detectors also work using one of these technologies or something different?

The FAA has a really nice guide.

What make you think smoke detectors would be different in planes? If so, why would we not use airplane smoke detectors on the doing l ground?

@Aron I think the question is along the lines of "Of the various types of smoke detectors, which type(s) are used on aircraft?"

The typical aircraft smoke detector works using optical sensing which exploits the Tyndall light scattering effect.
From Boeing:

(Source)
An LED illuminates a sample of air while a light sensor, which is not in line-of-sight with the LED, looks at the same sample of air. If enough light is scattered by smoke particles then the detector will signal an alarm.

I was doing some research and one FAA doc says, it works on particle sensing. So, now i am confused. Do you have any reference which says it works on optical sensing.

In essence the two are the same. The particles are sensed by their scattering effect on the light. I am the reference... I was at one time the chief mechanical engineer at a company that specializes in aircraft fire protection.

ahok, good to know that. I was looking at Kidde products on this and i think they have some similar smoke detectors.


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Are certain NOPAC routes reserved for military use?
Are any Northern Pacific (NOPAC) routes reserved for military use?


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Is it possible to take a liquid/gas inside aircraft cabin to contaminate the air?
As per the security rules, i think upto 100ml of liquid ( medicines, perfumes etc) can be taken on-board by a passenger but is it possible that, that 100ml can be converted to fumes/gas and contaminate the air inside the aircraft?
Any such incident has ever been reported?

100 mL of myriad substances could easily stink up the entire plane, among other things. A few notorious chemicals from biology labs spring to mind: TEMED (an amine that smells like rotten fish) and beta-mercaptoethanol (a thiol that smells like rotten eggs), and neither of those are particularly special.

Surströmming is more than capable of stinking up a whole airliner. This is the reason why it is forbidden on some airlines. And you can eat it too.

The durian fruit (found mostly in SE Asia) can easily stink up any modern airliner. It is banned in most hotels due to its strong smell. So not sure why the hate on an a can of air.

To answer your first question: it depends what you mean by "contaminate". The air in an aircraft cabin is constantly being renewed every two or three minutes (source (Airbus) - Boeing aircraft are similar), and the airflow pattern is such that the air only flows "across" the cabin, not along it. In other words, any contaminant you introduce will only reach the passengers seated in the same row as you (and perhaps the rows directly behind and in front of you), and will only stay there for a few minutes before the air exchange carries it overboard.
About 50% of the air which leaves the cabin is recycled back into the cabin, but it passes through filters first, which will stop any particles but will let gases through. Statistically, the concentration of whatever was introduced will drop to nearly zero after two passes through the ventilation system (due to dilution with fresh outside air).
My understanding is that the "100ml" rule is more designed to stop passengers from building liquid explosives aboard an aircraft (source (TSA)) than to stop them introducing harmful contaminants into the air.
I don't know of any incidents where an airborne substance was used maliciously in an airplane cabin, but you would get a similar effect through an on-board fire. The aircraft's ventilation system is designed to clear the cabin and flight deck of smoke within a short period of time (often by disabling the air recirculation system and increasing the fresh air flow into the aircraft).

"Statistically, the concentration of whatever was introduced will drop to nearly zero after two passes through the ventilation system" Huh? If each pass recycles 50% of the air, then two passes will still leave 25% of the contamination. That's not even close to zero.

...assuming the air is fully contaminated, of course.

Gases
For it to be converted to a gas, it would have to either be stored in a pressurized container or be a liquid that happens to vaporize at room temperature in the relatively small pressure difference between the surface and the cabin pressure altitude. Pressurized gas containers are not allowed on board U.S. airline flights unless they are:

Medical oxygen cylinders with in-tact and unmodified regulator valves,
Part of a life vest, or
Verifiably empty.

From the TSA Prohibited Items List:
Allowed in checked or carry-on luggage:

Small compressed gas cartridges
  (Up to 2 in life vests and 2 spares. The spares must accompany the life vests and presented as one unit)

Not allowed in any luggage:

Fire extinguishers and other compressed gas cylinders

From the "Can I bring my...?" search blank on the same page with the search term "compressed gas cylinder":

Except for personal medical oxygen cylinders, you can only carry an EMPTY compressed gas cylinder on board a plane. To be permitted (in either carry-on or checked baggage), it must be clearly visible to the Transportation Security Officer (TSO) that the cylinder is empty. Personal oxygen is permitted if the regulatory valve has not been tampered with or removed.

Aerosols
Aerosols are allowed, but are subject to the same 3-1-1 rule as other liquids, that is, they must be in a 100 mL or less container which is then stored inside a 1 L / 1 qt. zip-top bag. As Lightsider mentioned, these will almost certainly be filtered out of the air by the air filters in the ventilation system, as they are not in gaseous form (and likely are relatively large particles and/or molecules.)
From the same TSA page using the search term "aerosols," you get the following:

You may carry liquids, gels and aerosols in your carry-on bags only if they adhere to the 3-1-1 rule: containers must be 3.4 ounces or less; stored in a 1 quart/liter zip-top bag; 1 zip-top bag per person, placed in the screening bin. Larger amounts of non-medicinal liquids, gels, and aerosols must be placed in checked baggage.

If you have the right substances, it's possible.
In theory you'd only need 50 grams of Sarin to reach LD50 in a 747. Poor dispersion will mean much less than a 50% casualty rate, but 100 ml is enough to cause some damage even just spilled - it's volatile and evaporates on its own.
As the Aum Shinrikyo attack has shown, it's possible to produce privately. But a well-ventilated aircraft is probably not as good a place for chemical attacks as a crowded theater. Takes more time to get aid, though.
The 100ml rule is meant to protect against binary explosives, as well as innocent but dangerous liquids such as gasoline. Yes, an educated person apparently really decided it was worth bringing on a flight. Hull loss.

You don't even need to take a separate liquid.  Here is one example of a flight turning around due to a horrible smell.
 https://www.vice.com/en_us/article/bnpevw/one-year-on-still-thinking-about-poo-plane-303.  Here is another example, in which the man who was the source of the smell (necrotic tissue from an infection that happened while he was on vacation) later died.  http://www.foxnews.com/travel/2018/06/27/unbearable-smelling-passenger-that-caused-plane-s-emergency-landing-dies-from-tissue-necrosis.html.  There is also a SpiceJet incident similar to the first (British Airways) one.
But if one were intent on taking a liquid onboard to contaminate the air, yes, it would be possible, and there are certainly many extremely foul smelling or even toxic liquids that would evaporate if spread broadly enough.  Fortunately, the toxic compounds are difficult to obtain.
The air volume of a 747 is roughly 1,000 cubic meters.  (Other planes are much smaller, so easier to contaminate.)  When 100 ml of a liquid vaporizes, its volume depends on its density, temperature, pressure, and molecular weight.  The vaporized volumes of 100 ml of trimethyl amine, cadaverine, putrescine, or various sulfides would be within an order of magnitude of 10 liters, and 10 liters is 1 part in 1 million of 1,000 cubic meters.  Any of those would likely be foul smelling enough to require turning the plane around.  (It might be easier to just bring some rotting fish, though.)
Cabin air filters and the ventilation system would gradually reduce the concentration somewhat, but if they were truly effective, those example flights wouldn't have been cut short.  Ten passes through the system at 50% dilution per pass would reduce the original concentration by a factor of approximately 1,000, but that will take a while, and there are many compounds that stink even at 1 part in a billion.  (And once dozens of passengers have vomited, that will add butyric acid smell to the mix.)
If anyone figures out you brought the stench onboard, you probably risk being beaten and certainly prosecuted.  So yes, it's probably possible, but so far seems never to have occurred.


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How is fuel stored and routed on a trijet with one engine at the tail?
This question pertains to trijets with two engines on the wings and one at the tail, like the Lockheed Tristar. It does not pertain to trijets with all engines at the tail, like the Tu-154. (And it doesn't matter if the rear engine has an S-duct or not.)
How is fuel stored and routed on a trijet that has only one engine at the tail? Does the rear engine have totally separate fuel tanks and lines?

The fuel system needs to be able to supply any engine from any tank and ideally also move fuel between tanks, because otherwise an engine failure would lead to a shift in center of gravity. Mounting a tank far aft also requires the other tanks to be moved forward, but if you want to use the wings as tanks, this means shifting center of lift against center of gravity.